(1) Field of the Invention
The invention is related to connection means for a rotor blade to a rotor hub, particularly for a helicopter.
(2) Description of Related Art
Rotor blade connections to the rotor hub are highly loaded by centrifugal force and bending from lead/lag, flapping and torsion moments rendering the rotor blade connections actually one of the critical parts for function and properties of main rotors. In order to achieve a maximum of performance and safety, the general design target is to reduce the rotor blade connection to a minimum cross section for minimized drag and weight with fail safe features and damage tolerant failure mode.
It is known to connect the rotor hub to the rotor blade by means of two bolts arranged side by side in transverse direction to a longitudinal axis from the rotor blade to the rotor hub. Said arrangement of the bolts results in unfavourable stress distribution in the rotor blade connection. With two bolts side by side and offset from the longitudinal axis from the rotor blade to the rotor hub the direction of the force flow between the rotor hub and the rotor blade changes with the consequence of delamination in the neck of the blade connection. Said offset arrangement of the bolts calls for reinforced connection means with bigger cross sections and results consequently in bulky connection means with inappropriate aerodynamic drag.
The document FR 1 117935 A discloses a mounting device for a rotor blade of an aircraft. The rotor blade is fixed to a support linked to a rotor hub by means of a joint having an axis in line with the general orientation of the axis of rotation of the rotor.
The document US 2009155086 A1 discloses a rotorcraft rotor blade with a blade root and a free end opposite from the blade root, the blade being provided at least with a spar laid flat on a suction-side face and a pressure side-face of the blade, the main spar being secured to an attachment fitting at the blade root. The attachment fitting then includes first and second horizontal fastener bushings, the first and second fastener bushings being both perpendicular to the span of the blade and substantially perpendicular to an elevation direction (Z) that is substantially parallel to gravity (W), the elevation direction (Z) being perpendicular to the span.
The document U.S. Pat. No. 2,336,012 A discloses a propeller assembly, a hub, a blade having a root face of airfoil section and a shank member serving to mount the blade upon the hub, a transverse flange on the shank member for engaging the root end face of the blade.
The document GB 909617 A discloses a metal rotor blade having an extruded hollow spar forming the leading edge and having attached thereto a plurality of tapered pockets. The airfoil outboard portion of the spar is of substantially uniform cross-section and the inboard portion having varying internal dimensions and thinner top and bottom walls than the outboard portion, the inboard portion having flat upper and lower surfaces and gradually merging with the outboard portion. Root plates are bonded to the flat upper and lower surfaces; bolts holding the plates to the spar do not contact the plates. Load is transferred from the blade to the plates by the bonded joint, or should this joint fail then in shear by bolts. The plates are spaced apart by two spacers and a diaphragm and sealing strip seal the inboard end of the spar.
The document U.S. Pat. No. 4,340,335 A discloses a helicopter tail rotor of flexrotor or composite bearingless rotor design, a control wheel member being positioned in spaced relationship to the torque tube at the blade inner end and being connected thereto by pivotal push-pull rods connected at one of their ends to the control wheel member and at their opposite ends to the leading edge and trailing edge of the torque tube so that, as the control wheel member is selectively caused to rotate relative to the blade, one push-pull rod pulls upon the torque tube while the other push-pull rod pushes upon the torque tube to cause the torque tube to rotate, and hence the flexible spar to which it is attached to twist to thereby selectively change the blade pitch angle.
The document GB 656797 A discloses a non-metallic propeller blade being attached to a metallic blade root so as to allow slight radial and axial movements. Sheets of rubber are placed between the blade and the metal plate of the root, being vulcanized to both. Fastenings comprising a bolt held in conical washers in the blade and surrounded by a cylinder secured in the root plate, the intervening space being filled with rubber. In a modification the root plate is formed with transverse ribs and with interfitting recesses in the blade, the space between these being filled with rubber.